| OCR Text |
Show These equations can be solved by iterative procedures using a combination of Gauss-Seidel and Newton-Raphson methods to converge to the correct solution. Temperatures obtained from the energy balance are inserted into the surface heat balance equations to evaluate net heat flux to that surface. It should be pointed out that the steps up to the evaluation of total exchange areas gives a complete description of the radiation characteristics of the system (i.e. fixed furnace shape or size, wall emissivity, and gas compositions). Changes in temperatures, heat inputs etc., will show the effects on furnace performances of changes in the gas flowrate, flow pattern, enthalpies of gases, heat generation, and temperature distribution of the controlled parts of the furnace walls. 3.2 Discussion of Results The radiant section of the boiler has been zoned into 20 gas cubes and 48 surface zones, each having a characteristic dimension of 12 ft (3. 66m). This zone size has been used to give the most economic balance between computational requirements (storage and run-time), and the available flow and temperature data both for the tube walls and flame characteris tics. From the available da ta on burner pressure drops etc. it has been calculated that the flame length/burner for the existing burner configuration would be approximately 20-26 ft (6-8m), depending on the 'overlap' between adjacent corners, whilst for the proposed modification to the burner design, flame lengths would reduce to 16-20 ft (5-6m) due to improved air/fuel mixing and atomization. It is also important to note that in most high momentum turbulent diffusion flames approximately 80% of the fuel is burnt in the first 50% of the flame. The flow modelling data from the first part of this study was used to define the gas flow patterns in the radiant section of the boiler for both horizontal and angled burner posi tions. Figures 11 and 12 show these mass flow maps. This mass flow data also provides the basic information required to calculate the convective heat transfer rates to tubes from adjacent gas zones. Table 2 summarizes the fuel properties, and the products of combustion. This data provides the values of Rand R2 used in the evaluation of flue gas emissivity. |
